Age play is the term for consenting adults who roleplay in some way pertaining to age. Sometimes this is "infantilism", which is adults acting as babies. There are all ages.. this can also include schoolgirl or schoolboy with headmaster or headmistress, and every conceivable age in between. Consensuality is key, and let me stress again this is ADULTS - it has nothing whatsoever to do with anyone under the age of consenting adult.
Usually sexual roleplaying involving Daddy/daughter, Daddy/son, Mother/son, Mother/daughter, Police/arrested, Teacher/student, Doctor/nurse, Doctor/patient, Uncle/niece, etc. The main stipulation is that one person is either older or playing older than the second person, or the submissive person is younger or playing younger than the dominant. Both people involved are ALWAYS adults.
"Daddy" Chris and his "daughter" Amy decided to participate in ageplay even though Amy was over eighteen and not really his daughter.